Pre-treating aluminum surfaces with zirconium-and molybdenum-containing compositions

ABSTRACT

Provided herein is a process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to similarly pretreated, precoated workpieces, or to precoated parts. The parts are composed of steel, galvanized steel, and/or alloy-galvanized steel. Further provided herein is a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer, or by surface coating, wherein the workpieces are a) pickled by means of an aqueous, acidic solution comprising mineral acid by dipping or spraying, b) rinsed with water, and c) brought into contact with an aqueous, acidic, chromium-free solution comprising Zr as complex fluoride and Mo as molybdate by application by dipping or spraying. After subsequent drying, a layer weight from 2 to 15 mg/m 2  of Zr and Mo is formed.

The invention relates to a process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to similarly pretreated or optionally otherwise precoated workpieces or to optionally pretreated parts composed of steel and/or galvanized and/or alloy-galvanized steel and also for a subsequent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer or by surface coating.

For the chemical surface treatment of metals, for example as preparation for application of surface coatings, adhesives and polymers, processes in which the metal surface is cleaned in the first step, rinsed with water in the second step and finally wetted with an aqueous solution forming chemical conversion coatings in the third step and the liquid film is dried are known. In this way, a thin, nonmetallic coating which can, when the composition of the treatment liquid and the reaction conditions are appropriately selected, decisively improve the surface quality is formed on the metal. Thus, for example, coatings composed of surface coating compositions, adhesives and polymers, optionally in the form of films, can display significantly better adhesion and noticeably increased anticorrosion when they are applied to metal which has been pretreated in this way.

Processes of the abovementioned type employ, for example, an aqueous solution which comprises hexavalent chromium, trivalent chromium, alkali metal ions and silicon dioxide in particular ratios and produces coatings for electrical insulation, for anticorrosion and as adhesion base for surface coatings and the like (DE-B 17 69 582).

Due to the presence of hexavalent chromium, these processes all have the disadvantage that, in particular, precautionary measures are necessary for the application of the coating composition and the handling of the coated metal.

In order to avoid the disadvantages associated with the use of solutions comprising hexavalent chromium, a different category of processes provides for application of conversion coatings, especially for aluminum surfaces, treatment with compositions based on the fluoro anions of zirconium and/or titanium (U.S. Pat. No. 4,148,670, FR-A-942 789, EP-A-106 389, EP-A-825 280).

Recently, the use of workpieces having surfaces of aluminum or aluminum alloys in vehicle construction is becoming increasingly important. Significant reasons for this are reduction of the vehicle weight and also the advantageous recyclability of such workpieces. Here, it is generally customary to produce vehicles by joining together a plurality of parts which have generally previously been formed without cutting machining and consist of workpieces having surfaces of aluminum or aluminum alloys or else having surfaces of steel and/or galvanized steel and/or alloy-galvanized steel. By far the most important forms of joining or assembly are welding or adhesive bonding.

The joining of the parts is normally followed by a corrosion-protecting treatment which, depending on the nature of the joined parts, can consist of a phosphating treatment, a chromium-free conversion treatment, application of a primer or a surface coating.

To ensure defect-free joining of the parts having surfaces of aluminum or of aluminum alloys to one another or to parts composed of steel and/or galvanized or alloy-galvanized steel, it has to be ensured that the surfaces of the aluminum or of the aluminum alloy are free of aluminum oxide or oxides of possibly alloying constituents of the aluminum. A pickling treatment which seems attractive for this purpose does not lead to the objective since the aluminum surface becomes coated with a fresh oxide layer after a very short time.

In order to solve the problem outlined above, the process of EP-B-700 452 provides for surfaces of aluminum or alloys thereof to be brought into contact with an aqueous solution comprising complex fluorides of the elements boron, silicon, titanium, zirconium or hafnium either individually or in admixture with one another in total concentrations of the fluoro anions of from 100 to 4000 mg/l and having a pH of from 0.3 to 35 to effect pretreatment before a second permanent corrosion-protecting treatment. Between pretreatment and permanent corrosion-protecting conversion treatment, the parts composed of aluminum or alloys thereof are subjected to a noncutting and/or cutting shaping process and/or joined together or to parts composed of steel and/or galvanized and/or alloy-galvanized steel by adhesive bonding and/or welding. The application of the solution optionally comprising polymers of a particular nature can be effected by spraying, dipping or no-rinse methods, with in the case of the no-rinse method the amount of wet film preferably being in the range from 2 to 10 ml/m², preferably from 4 to 6 ml/m², of metal surface. Regardless of the way in which the solution is applied, it is advantageous to carry out drying at temperatures in the range from 40 to 85° C. For the purpose of cleaning, the parts composed of aluminum or alloys thereof are cleaned under acidic or alkali conditions before the first conversion treatment, with preference being given to carrying out further cleaning steps and intermediate rinses with water and/or with activating rinsing baths before the permanent corrosion-protecting treatment.

From the figures for the concentrations of fluoro anions in the solutions to be applied and the amount of wet film, it is possible to calculate, for the case of application of a fluorotitanate-comprising solution, an applied amount of from 0.06 to 11.73 mg/m², preferably from 0.12 to 7.04 mg/m² (in each case reported as titanium metal), and, for the case of application of a fluorozirconate-comprising solution, an applied amount of from 0.09 to 17.78 mg/m², preferably from 0.18 to 7.04 mg/m² (in each case reported as zirconium metal).

The studies preceding the conception of the present invention have shown that many of the process possibilities described in EP-B-700 452 lead to results which are not very advantageous, particularly in respect of the temporary anticorrosion achieved by the first chemical conversion treatment and the volume resistance which is important for producing welded joins.

Good adhesive bonding can be achieved by the process disclosed in U.S. Pat. No. 6,020,030 A, in which an organophosphorus compound is used for pretreating aluminum substrates. However, the process is susceptible to bath contamination by aluminum ions which therefore have to be removed by means of an ion-exchange resin.

It is an object of the invention to provide a process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding, in particular for joining by adhesive bonding, to similarly pretreated or optionally otherwise precoated workpieces or to parts composed of steel and/or galvanized and/or alloy-galvanized steel, in particular to similarly pretreated workpieces, and also a subsequently permanent anticorrosion treatment by phosphating and/or by means of a chromium-free conversion treatment or by application of primer and/or by surface coating, in particular by phosphating and a chromium-free conversion treatment or by surface coating, which normally leads to workpieces having a sufficiently low volume resistance combined with good phosphatability and adhesion.

In addition, the process of the invention should preferably lead to good temporary anticorrosion values and be largely insensitive to contamination of the bath by aluminum ions.

The object is achieved firstly by the process of the type mentioned at the outset being configured according to the invention so that the workpieces are

-   -   a) pickled by means of an aqueous, acidic solution comprising         mineral acid by dipping or spraying,     -   b) rinsed with water and     -   c) brought into contact with an aqueous, acidic solution which         is chromium-free and comprises Zr as complex fluoride and Mo as         molybdate in a weight ratio (calculated as Zr/Mo metal) of Zr:Mo         of from 15:1 to 3.5:1 by application by dipping or spraying so         as to result, after subsequent drying, in a layer weight of in         each case from 2 to 15 mg/m² of Zr and Mo, where the solution         comprises from 100 to 800 mg/l of Zr and from 30 to 100 mg/l of         Mo (calculated as Zr/Mo metal) and has a pH of from 2.5 to 4.5.

In the application by dipping or spraying, it has been found that neither in the application of solutions comprising only fluorozirconate or only molybdate nor in the application of solutions in which the weight ratio (calculated as Zr/Mo metal) of zirconium:molybdenum is outside the weight ratio of from 15:1 to 3.5:1 nor in the application of solutions which lead to layer weights of zirconium and molybdenum outside the range from 2 to 15 mg/m² are results which are satisfactory in respect of the object of the invention achieved.

The aqueous, acidic solution in step c) for application by dipping or spraying preferably comprises Zr and Mo in a weight ratio of from 15:1 to 5:1, particularly preferably from 13:1 to 7:1 and very particularly preferably from 11:1 to 9:1.

The application by dipping or spraying preferably results, after drying, in a layer weight of in each case from 2 to 12 mg/m², particularly preferably from 2 to 10 mg/m² and very particularly preferably from 2 to 8 mg/m², of Zr and Mo.

The aqueous solution for application by dipping or spraying preferably comprises from 250 to 700 mg/l of Zr and from 30 to 80 mg/l of Mo, particularly preferably from 400 to 600 mg/l of Zr and from 40 to 60 mg/l of Mo and very particularly preferably from 475 to 525 mg/l of Zr and from 45 to 55 mg/l of Mo.

Furthermore, the aqueous solution for application by dipping or spraying preferably has a pH of from 3.1 to 4.3 and particularly preferably from 3.6 to 4.0 and also preferably has a temperature of from 20 to 50° C. and particularly preferably from 20 to 30° C.

Secondly, the object is achieved by the process of the type mentioned at the outset being configured according to the invention so that the workpieces are

-   -   a) pickled by means of an aqueous, acidic solution comprising         mineral acid by dipping or spraying,     -   b) rinsed with water and     -   c) brought into contact with an aqueous, acidic solution which         is chromium-free and comprises Zr as complex fluoride and Mo as         molybdate in a weight ratio (calculated as Zr/Mo metal) of Zr:Mo         of from 2:1 to 1:2 by the roller application method so as to         result, after subsequent drying, in a layer weight of in each         case from 2 to 15 mg/m² of Zr and Mo, where the solution         comprises from 0.4 to 7.5 g/l of Zr and from 0.4 to 7.5 g/l of         Mo (calculated as Zr/Mo metal) and has a pH of from 1.0 to 3.0.

In the application by the roller application method, it has been found that neither in the application of solutions comprising only fluorozirconate or only molybdate nor in the application of solutions in which the weight ratio (calculated as Zr/Mo metal) of zirconium:molybdenum is outside the weight ration of from 2:1 to 1:2 nor in the application of solutions which lead to layer weights of zirconium and molybdenum outside the range from 2 to 15 mg/m² are results which are satisfactory in respect of the object of the invention achieved.

The aqueous, acidic solution in step c) for application by the roller application method preferably comprises Zr and Mo in a weight ratio of from 1.7:1 to 1:1.7, particularly preferably from 1.4:1 to 1:1.4 and very particularly preferably from 1.1:1 to 1:1.1.

The application by the roller application method preferably results, after drying, in a layer weight of in each case from 2 to 12 mg/m², particularly preferably from 2 to 10 mg/m² and very particularly preferably from 2 to 8 mg/m², of Zr and Mo.

The aqueous solution for application by the roller application method preferably comprises from 1.0 to 6.0 g/l of Zr and from 1.0 to 6.0 g/l of Mo, particularly preferably from 2.0 to 4.0 g/l of Zr and from 2.0 to 4.0 g/l of Mo and very particularly preferably from 2.8 to 3.2 g/l of Zr and from 2.8 to 3.2 g/l of Mo.

Furthermore, the aqueous solution for application by the roller application method preferably has a pH of from 1.4 to 2.7 and particularly preferably from 1.8 to 2.5.

As regards the defined layer weight of zirconium and molybdenum (from 2 to 15 mg/m²) to be set in each case, it is of critical importance that, depending on the manner of application, namely application by spraying or dipping or application by the roller application method, aqueous, acidic solutions which differ in respect of concentration and ratio of the fluorozirconate and molybdate anions and the pH are used in step c).

The aqueous, acidic solution in step c) can be provided by prior dilution of an appropriate concentrate, preferably by a factor of from 1:30 to 1:100, more preferably by a factor of about 1:50, preferably with water and optionally setting of the pH.

To attain the objective pursued by the invention, it is also important for the workpieces to be pickled by means of an aqueous, acidic solution comprising a mineral acid by dipping or spraying. Alkali cleaning results, for example, in the formation of zirconium/molybdenum layers having poor volume resistances.

The joining of the workpieces which have been pretreated according to the invention can be to similarly pretreated or optionally otherwise precoated, e.g. phosphated, workpieces, to surfaces of aluminum or alloys thereof. If joining to parts composed of steel and/or galvanized and/or alloy-galvanized steel is intended, these parts can have bare or precoated surfaces. One suitable precoat can be, for example, a phosphate layer having a layer weight of not more than 2 g/m² or a layer of a conductive primer.

If the workpieces are oily, the pickling process should be preceded by a cleaning/degreasing step or the pickling process should be carried out in such a way that simultaneous cleaning/degreasing occurs. The latter can be achieved by addition of surfactant to the pickling solution.

As processes for phosphating treatment, processes which work with solutions based on zinc phosphate, in particular corresponding to the low-zinc technology, or with alkali metal phosphate are in the foreground. The solutions can have been modified by addition of further small amounts of polyvalent cations such as calcium, magnesium, nickel, copper or manganese.

For the chromium-free conversion treatment, use is made in particular of acidic solutions of the fluoro complexes of titanium, zirconium, hafnium or else silicon, optionally with a content of an organic polymer.

These acidic solutions can additionally comprise at least one organosilane and/or at least one hydrolysis product thereof and/or at least one condensation product thereof.

The at least one organosilane preferably has at least one amino group. Particular preference is given to a bis(trimethoxysilylpropyl)amine or an organosilane which can be hydrolyzed to an aminopropylsilanol and/or to 2-aminoethyl-3-aminopropylsilanol.

As primer, it is possible to apply reactive primers or bonding agents.

The pretreatment according to the invention of the workpieces ensures satisfactory, temporary anticorrosion for relative long storage times. During this time, no adverse effects on the weldability, in particular for electric resistance welding, or the adhesive bondability occur. In addition, in respect of the weldability, it is ensured that the volume resistance is virtually identical on all surface regions of the workpiece.

Workpieces for the purposes of the present invention are strip, sheet and individual parts such as profiles.

The application of the solution as per step c) can be effected by spraying or dipping, in each case with or without rinsing with water. In the case of application without rinsing with water, it is advantageous to remove excess treatment solution by means of squeegee rollers.

In the treatment of sheet or strip, application of the treatment solution by the roller coating method is particularly advantageous. It allows defined setting of the desired wet film thickness in a single operation.

After the above-described types of application of the solution, the workpiece is dried or the solution is evaporated. Object temperatures of from 30 to 90° C. are particularly advantageous.

To make up the treatment liquids, use is usually made of concentrates which are diluted with low-salt water, preferably deionized water, to the concentrations to be set in each case. To avoid introduction of alkali metal ions, it is particularly advantageous to introduce the required fluoro anions of zirconium by means of the free acid and to set the respective pH, where necessary, by addition of ammonia. On the other hand, the molybdate is advantageously introduced as ammonium heptamolybdate and/or sodium heptamolybdate, preferably as ammonium heptamolybdate and particularly preferably as ammonium heptamolybdate×7H₂O.

For the purpose of the present invention, the term “molybdate” also comprises protonated forms such as, in particular, molybdic acid.

The pickling of the workpieces (process step a)) is effected using an aqueous, acidic solution comprising mineral acid. It can be carried out electrolytically or chemically. In the case of electrolytic pickling, phosphoric acid is particularly suitable as mineral acid. Pickling by a chemical route, which is generally preferred because of the simpler mode of operation in terms of apparatus, can be carried out using nitric acid or nitric acid/hydrofluoric acid. In a preferred embodiment of the invention, the workpieces are pickled by spraying or dipping using a solution comprising surfactant, sulfuric acid and a compound selected from the group consisting of hydrofluoric acid, phosphoric acid and iron(III) sulfate, preferably hydrofluoric acid, with solutions comprising from 3 to 8 g/l of sulfuric acid, from 50 to 150 mg/l of uncomplexed, free fluoride and from 1 to 3 g/l of nonionic surfactant having been found to be particularly suitable. Ethylene oxide adducts with fatty alcohols and, for example, abietic acid are particularly suitable as nonionic surfactants.

Measurement of the free fluoride was carried out using a fluoride-sensitive electrode, with calibration of the electrode being carried out using solutions whose pH was identical to that of the solution to be tested.

In order to obtain layers having an optimal volume resistance in the subsequent treatment as per step c), the pickling process should be carried out so that a removal of metal of from about 0.1 to 0.6 g/m² of workpiece service is obtained.

The rinsing with water following the pickling of the workpieces, corresponding to step b), is preferably carried out in a plurality of rinsing stages, and it is particularly advantageous to convey the rinsing water in a cascade-like manner in countercurrent to the workpiece. Here, the last rinsing stage should be carried out using deionized water. The treatment as per step c) following the pickling and rinsing step prevents renewed growth of an oxide layer occurring on the workpieces having a surface of aluminum or aluminum alloy.

In an advantageous embodiment of the invention, the solution used in step c) additionally comprises at least one polymer selected from the group consisting of poly(meth)acrylic acid, (meth)acrylic acid copolymers, polyvinylphosphonic acid, vinylphosphonic acid copolymers and maleic acid copolymers.

Preference is given to using (meth)acrylic acid-maleic acid copolymers as (meth)acrylic acid copolymers and vinylphosphonic acid-acrylic acid copolymers as vinylphosphonic acid copolymers. Particularly suitable polymers are polyacrylic acid and acrylic acid copolymers and in the case of the latter especially acrylic acid-maleic acid copolymers.

The poly(meth)acrylic acid used preferably has a number-average molecular weight (MW) in the range from 4000 to 300 000 g/mol, particularly preferably from 50 000 to 250 000 g/mol and very particularly preferably from 100 000 to 250 000 g/mol.

The (meth)acrylic acid copolymer used preferably has a number-average molecular weight (MW) in the range from 4000 to 100 000 g/mol and particularly preferably from 60 000 to 80 000 g/mol.

The polyvinylphosphonic acid used or the vinylphosphonic acid copolymer used preferably has a number-average molecular weight (MW) in the range from 4000 to 70 000 g/mol and particularly preferably in the range from 10 000 to 30 000 g/mol.

The concentration of the at least one polymer is in the range from 100 to 600 mg/l, preferably from 100 to 400 mg/l, particularly preferably from 135 to 290 mg/l and very particularly preferably from 170 to 180 mg/l. The use of the at least one polymer makes it possible to achieve a layer weight of Zr and Mo in the target range from in each case 2 to 15 mg/m² in application by spraying, relatively independently, preferably largely independently, of the spraying time. This is advantageous especially because similar layer weights can be achieved even at different strip speeds. Adverse effects on the volume resistance due to the polymer content are, on the other hand, not observed.

In the case of a subsequent treatment by noncutting forming after step c), a further advantageous embodiment of the invention provides for a lubricant to be applied to the workpieces. Such lubricants are, in particular, forming oils based on mineral oil, which can be fully synthetic or of natural origin, or dry lubricants based on polyethylene/polyacrylate.

It is generally useful to insert cleaning stages and rinsing-with-water stages before the permanent anticorrosion treatment. In the case of an envisaged application of primer or a surface coating, prior drying is advisable. If a chromium-free conversion treatment is to follow, the application by dipping or spraying can also be carried out wet-on-wet, i.e. without prior drying. In the case of application by the roller coating method, intermediate drying is indispensable. In the case of a subsequent phosphating treatment, which can likewise be carried out wet-on-wet, it is advantageous to provide an activation treatment, for example using a titanium- and phosphate-comprising activating agent.

The process of the invention normally gives workpieces having layers which allow defect-free forming and/or adhesive bonding or, as a result of the low electrical volume resistance which is uniform over the workpiece surface, defect-free and problem-free welding. In addition, the workpieces are highly suitable for a subsequent permanent anticorrosion treatment.

The invention will be illustrated in more detail with the aid of the following examples, which are not to be interpreted as constituting a limitation.

EXAMPLES

With the exception of a metal sheet which was cleaned under alkaline conditions (comparative example CE2 in Tab. 1), sheets composed of aluminum alloys of the grades AA 6111 and AA 5754 were firstly subjected to degreasing pickling at a temperature of 50° C. by dipping or spraying. The pickling solution comprised 6 g/l of sulfuric acid (100% strength), 100 mg/l of hydrofluoric acid (100% strength) and 2 g/l of nonionic surfactant consisting of an ethoxylated fatty alcohol and ethoxylated abietic acid in a weight ratio of 1:1.

The pickling process was carried out in such a way that the removal of material during pickling was from 0.05 to 0.2 g/m² in the case of the alloy AA 5754 and was from 0.05 to 0.4 g/m² in the case of the alloy AA 6111. Treatment times in the range from 5 to 20 seconds were required for this purpose.

The workpieces were subsequently rinsed thoroughly with water, in the last stage with deionized water. The volume resistances measured on the single sheet were about 60 pohm in the case of the alloy AA 5754, and about 13 pohm in the case of the alloy AA 6111.

This was followed by a treatment with solutions of hexafluorozirconic acid and/or of molybdate which in each case optionally comprised a polymer or copolymer and whose data in respect of the concentration of zirconium and/or molybdenum and also of (co)polymer, the pH and the application temperature are shown in Tab. 1, for 6 seconds by spray application. If necessary, the pH was adjusted using ammonia solution. Excess treatment solution was removed by means of squeegee rollers and the treated surface was subsequently dried.

The polymers A to D in Tab. 1 were the following:

A: Polyacrylic acid, MW=about 60 000 g/mol in colloidal solution, B: Acrylic acid-maleic acid copolymer, MW=about 70 000 g/mol, C: Polyacrylic acid, MW=about 250 000 g/mol, D: Vinylphosphonic acid-acrylic acid copolymer, MW=4000 to 70 000 g/mol.

TABLE 1 Concentration in mg/l (Comp.) ZrF₆ ²⁻ + Temp. Ex. Zr Mo ZrF₆ ²⁻ MoO₄ ²⁻ MoO₄ ²⁻ Polymer pH in ° C. CE1 500 50 1125 83 1208 A: 250 3.6 50 E1 500 50 1125 83 1208 A: 250 3.6 30 E2 500 50 1125 83 1208 B: 500 3.6 30 E3 500 50 1125 83 1208 C: 175 3.6 23 E4 500 50 1125 83 1208 D: 250 3.6 23 E5 300 80 675 132.8 807.8 C: 175 3.9 23 E6 600 40 1350 66.4 1416.4 C: 350 3.6 23 CE2 500 50 1125 83 1208 C: 175 3.6 23 E7 500 50 1125 83 1208 0 3.6 23 CE3 500 50 1125 83 1208 C: 175 2.1 23 CE4 500 25 1128 41.5 1166.5 C: 175 3.6 23 CE5 100 50 225 83 308 C: 175 3.6 23

TABLE 2 Layer weight (mg/m²) Volume resistance (μohm) (Comp.) AA 6111 AA 5754 AA 6111 AA 5754 Ex. Zr Mo Zr Mo 0 d 30 d 0 d 30 d CE1 26 4 37 7 25 38 62 100 E1 9 5 14 6 8 10 9 11 E2 2 6 5 7 9 11 13 15 E3 4 8 8 7 12 14 13 15 E4 3 5 10 6 7 8 10 12 E5 3 5 8 8 10 14 4 16 E6 2 1 4 2 13 14 5 7 CE2 7 12 8 3 26 25 18 33 E7 7 4 9 6 15 15 6 15 CE3 18 8 24 12 67 73 40 79 CE4 6 4 8 6 26 44 9 21 CE5 4 3 8 8 19 22 9 18

TABLE 3 Phosphatability Modified APGE test (Comp.) (+/∘/−) (cycles survived) Ex. AA 6111 AA 5754 AA 6111 AA 5754 CE1 n.m. n.m. 10-30 10-30 E1 + + ≥45 ≥45 E2 + + ≥45 ≥45 E3 + + ≥45 ≥45 E4 + + ≥45 ≥45 E5 + + ≥45 ≥45 E6 + + ≥45 ≥45 CE2 + — n.m. n.m. E7 + ∘ ≥45 ≥45 CE3 — — n.m. n.m. CE4 ∘ — n.m. n.m. CE5 ∘ — n.m. n.m. n.m. = not measured

Column 2 in Tab. 2 shows the layer weights of Zr and of Mo achieved in this way in mg/m². The measurement of the individual layer weights was carried out by means of X-ray fluorescence (XRF) analysis.

In addition, the volume resistances in pohm obtained in the individual measurements of the metal sheets are shown in column 3 of Tab. 2. Measurement of the volume resistances was carried out immediately after drying/evaporation (in each case first row “0 d”) and after storage for 30 days (in each case second row “30 d”). It was carried out in accordance with leaflet 2929 (of September 2001) of the Deutscher Verbandes für Schweißen and verwandte Verfahren e.V. (DSV) using copper electrodes having a diameter of 20 mm.

The corresponding phosphatabilities and the results of the modified APGE test described below in cycles survived are given in columns 2 and 3 of Tab. 3.

The adhesion was determined by means of a modified APGE (Arizona Proving Ground Equivalent) test. For this purpose, two test plates (each 56.25×25×0.25 mm) in each case were coated with an industrial dry lubricant and adhesively bonded by means of a suitable industrial adhesive. Six such pairs of test plates were then screwed together at their respective ends to form a chain which was subjected to a tensile stress of 2400 N. For each week of the test, the following program of conditions was used:

-   -   1. Dipping for 15 minutes into a 5% strength by weight solution         of NaCl in distilled water,     -   2. Drying for 105 minutes in a dry environment,     -   3. 22 hours at controlled temperature and atmospheric humidity:         50° C. and 90% relative atmospheric humidity,     -   4. 4 repetitions of steps 1 to 3, and     -   5. 48 hours at 50° C. and 90% relative atmospheric humidity.

One sequence of the steps 1 to 3 represents, by definition, one cycle. A cycle is in each case considered to have been survived when the adhesive bonding between all test plates of the chain stands fast. The test overall is counted as having been passed when at least 45 cycles have been survived.

The phosphatability was determined with the aid of scanning electron micrographs. Here, “+” in Table 3 means a closed, finely crystalline phosphate layer, “o” means a closed, coarsened phosphate layer (crystal having an edge length of >20 μm) and “−” means a phosphate layer which is not closed through to not present.

The measured values in Tab. 2 and 3 allow the following to be concluded: the treatment of the aluminum sheets with an alkaline cleaner carried out to determine the necessity of an acidic pickling treatment in step a) in comparative example CE2 leads to sheets having poor volume resistances (18 pohm for AA 5754 and 26 pohm for AA 6111).

It can be seen from comparative example CE1 that an excessively high Zr layer weight of the layer obtained in the treatment according to step c) (37 mg/m² in the case of AA 5754 and 26 mg/m² in the case of AA 6111) results in very high volume resistances being obtained, especially after storage for 30 days (100 pohm in the case of AA 5754 and 38 pohm in the case of AA 6111).

In the case of comparative examples CE4 and CE5, the layer weights obtained are in the desired range, but as a result of the zirconium/molybdenum ratio in the treatment solution for process step c) of 20:1 (see CE4) or 2:1 (see CE5) the volume resistances obtained are unacceptable, especially for AA 6111 (26 and 44 pohm in the case of CE4 and 19 and 22 pohm in the case of CE5).

In the case of comparative example CE3 having a pH which is too low, namely 2.1, undesirably high applied weights of Zr (24 mg/m² for AA 5754 and 18 mg/m² for AA 6111) and very much too high volume resistances (40 and 79 pohm for AA 5754 and 67 and 73 pohm for AA 6111) are observed.

In comparison, examples E1 to E7 show that when the conditions essential to the invention in respect of the type of pickling treatment, the Zr/Mo ratio, the layer weight produced, the respective concentration and the pH ranges of the treatment solutions are adhered to, layers having extremely good volume resistances combined with good adhesion properties are obtained.

In addition, it can be seen from Tab. 3 that all examples E1 to E7 have passed the modified APGE test, i.e. have in each case survived at least 45 cycles, both in the case of AA 5754 and also in the case of AA 6111, while this clearly does not apply in the case of comparative example CE1 with in each case only from 10 to 30 cycles survived. The other comparative examples CE2 to CE5 were no longer subjected to the modified APGE test because of the poor results already obtained for the layer weight or volume resistance (Tab. 2, see above).

With regard to the phosphatability (see Tab. 3), all examples E1 to E7 (with the exception of E7 for AA 5754) always display a closed, finely crystalline phosphate layer (“+”), while comparative examples CE2 to CE5 in the case of AA 5754 merely have a phosphate layer which is not closed through to not present (“−”) and (with the exception of CE2) also show significantly poorer results in the case of AA 6611.

The test plates of examples E6 and E7 which have been pretreated according to the invention and also an unpretreated bare test plate CE6 were also subjected to a multistage anticorrosion treatment consisting of the following steps:

i) Alkaline cleaning (60° C.; 180 s) ii) Rinsing (mains water; RT, 60 s) iii) Activation (titanium phosphate; RT, 30 s) iv) Phosphating (trications; 53° C., 180 s) v) Rinsing (mains water; RT, 30 s) vi) Passivation (zirconium fluoride; RT, 45 s) vii) After-rinsing (demineralized water; RT, 30 s) viii) Drying (convection oven; 100° C.; 7 min)

After subsequent electrophoretic coating and topcoating, a filiform corrosion test in accordance with DIN EN 3665 (average values in accordance with DIN EN ISO 4628-8) and a cyclic corrosion test in accordance with VDA 621-415 (average values in accordance with DIN EN ISO 4628-8) were each carried out. The smaller the measured migration under the coating in mm, the better was the anticorrosion. The results are shown in the following table.

TABLE 4 Filiform VDA (Comp.) (mm) (mm) Ex. AA 6111 AA 5754 AA 6111 AA 5754 CE6 0.8 0.1 0.1 0.1 E6 0.8 0.1 0.1 0.1 E7 1.0 0.1 0.1 0.1

The anticorrosion in the case of E6 and E7 is in each case comparable to that in the case of CE6. The pretreatment according to the invention thus does not have an adverse effect on the anticorrosion achieved subsequently by means of an anticorrosion treatment. 

1. A process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to pretreated workpieces, precoated workpieces, or precoated parts composed of steel and/or galvanized steel and/or alloy-galvanized steel, and also for a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer or by surface coating, wherein the process comprises: a) pickling the workpieces using an aqueous, acidic solution comprising mineral acid by one of dipping and spraying; b) rinsing the workpieces with water; c) applying an aqueous, acidic, chromium-free solution to the workpieces by one of dipping and spraying, the solution comprising: Zr as complex fluoride, and Mo as molybdate, wherein a weight ratio, calculated as Zr/Mo metal, of Zr:Mo is from 15:1 to 3.5:1, and wherein the solution includes from 100 to 800 mg/l of Zr and from 30 to 100 mg/l of Mo, calculated as Zr/Mo metal, and has a pH from 2.5 to 4.5; and d) drying the workpieces, wherein, after the drying, a layer weight is provided on each workpiece of from 2 to 15 mg/m² of Zr and Mo.
 2. The process according to claim 1, wherein the aqueous, acidic, chromium-free solution in step c) comprises Zr as complex fluoride and Mo as molybdate in the weight ratio, calculated as Zr/Mo metal, of Zr:Mo of from 13:1 to 7:1.
 3. A process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to pretreated workpieces, precoated workpieces, or precoated parts composed of steel, galvanized steel, and/or alloy-galvanized steel, and also for a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer or by surface coating, wherein the process comprises: a) pickling the workpieces using an aqueous, acidic solution comprising mineral acid by dipping or spraying; b) rinsing the workpieces with water; c) applying an aqueous, acidic, chromium-free solution to the workpieces dipping or spraying, the solution comprising: Zr as complex fluoride, and Mo as molybdate, wherein a weight ratio, calculated as Zr/Mo metal, of Zr:Mo is from 2:1 to 1:2, and wherein the solution includes from 0.4 to 7.5 g/l of Zr and from 0.4 to 7.5 g/l of Mo, calculated as Zr/Mo metal, and has a pH from 1.0 to 3.0; and d) drying the workpieces, wherein, after the drying, a layer weight is provided on each workpiece of from 2 to 15 mg/m² of Zr and Mo.
 4. The process according to claim 3, wherein the aqueous, acidic, chromium-free solution in step c) comprises Zr as complex fluoride and Mo as molybdate in the weight ratio, calculated as Zr/Mo metal of Zr:Mo of from 1.4:1 to 1:1.4.
 5. The process according to claim 1, wherein the aqueous, acidic, chromium-free solution in step c) further comprises at least one polymer selected from the group consisting of poly(meth)acrylic acid, (meth)acrylic acid copolymers, polyvinylphosphonic acid, vinylphosphonic acid copolymers, and maleic acid copolymers.
 6. The process according to claim 5, wherein the at least one polymer is polyacrylic acid and/or an acrylic acid-maleic acid copolymer.
 7. The process according to claim 1, wherein the workpieces are pickled using a solution comprising surfactant, hydrofluoric acid, and sulfuric acid.
 8. The process according to claim 7, wherein the workpieces are pickled using the solution further comprising from 3 to 8 g/l of the sulfuric acid, from 50 to 150 mg/l of free fluoride, and from 1 to 3 g/l of a nonionic surfactant.
 9. The process according to claim 1, further comprising pretreating the workpieces for noncutting forming, the pretreating comprising applying a lubricant before the noncutting forming.
 10. The process according to claim 1, wherein said cleaning and said rinsing are carried out before said corrosion-protecting.
 11. An aqueous acidic solution for the pretreatment of workpieces having a surface of aluminum or aluminum alloys, wherein the solution is chromium-free and comprises Zr as complex fluoride and Mo as molybdate in a weight ratio calculated as Zr/Mo metal of Zr:Mo of from 15:1 to 3.5:1, and wherein the solution further comprises from 100 to 800 mg/l of Zr and from 30 to 100 mg/l of Mo calculated as Zr/Mo metal and has a pH of from 2.5 to 4.5.
 12. A concentrate from which a solution according to claim 11 is obtainable by dilution with water or an aqueous solution.
 13. An aqueous acidic solution for the pretreatment of workpieces having a surface of aluminum or aluminum alloys, wherein the solution is chromium-free and comprises Zr as complex fluoride and Mo as molybdate in a weight ratio calculated as Zr/Mo metal of Zr:Mo of from 2:1 to 1:2, and wherein the solution further comprises from 0.4 to 7.5 g/l of Zr and from 0.4 to 7.5 g/l of Mo calculated as Zr/Mo metal and has a pH of from 1.0 to 3.0.
 14. A concentrate from which a solution according to claim 13 is obtainable by dilution with water or an aqueous solution.
 15. A workpiece comprising a surface of aluminum or aluminum alloys, wherein the workpiece has been pretreated by means of a process according to claim
 1. 16. The process according to claim 10, further comprising applying an activation treatment prior to said corrosion-protecting.
 17. The concentrate according to claim 12, wherein the solution is further obtainable through the adjustment of pH.
 18. The concentrate according to claim 14, wherein the solution is further obtainable through the adjustment of pH. 